U.S. patent number 4,285,396 [Application Number 06/006,415] was granted by the patent office on 1981-08-25 for steam generator tube support system.
This patent grant is currently assigned to Wachter Associates, Inc.. Invention is credited to Frank Schwoerer, William J. Wachter.
United States Patent |
4,285,396 |
Schwoerer , et al. |
August 25, 1981 |
Steam generator tube support system
Abstract
A system for supporting a bundle of tubes against lateral
movement in a heat exchanger, such as a nuclear power plant steam
generator, wherein spring collar devices positioned at intervals
along the tubes permit lateral movement of the tubes but prevent
any relative movement between the tube and its associated spring
collar device. Any lateral movement is between adjacent spring
collar devices. With this arrangement, fretting and corrosion
between the outer surfaces of the tubes and their lateral support
members are substantially eliminated.
Inventors: |
Schwoerer; Frank (Chevy Chase,
MD), Wachter; William J. (Wexford, PA) |
Assignee: |
Wachter Associates, Inc.
(Gibsonia, PA)
|
Family
ID: |
21720766 |
Appl.
No.: |
06/006,415 |
Filed: |
January 25, 1979 |
Current U.S.
Class: |
165/162; 122/511;
138/113; 138/114; 165/69; 165/82; 165/DIG.409; 122/510 |
Current CPC
Class: |
F28F
9/013 (20130101); Y10S 165/409 (20130101) |
Current International
Class: |
F28F
9/013 (20060101); F28F 9/007 (20060101); F28D
007/00 (); F22B 037/24 () |
Field of
Search: |
;122/510,511,DIG.13,235K
;165/69,82,162,172 ;285/133,138 ;138/113,114 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Yuen; Henry C.
Attorney, Agent or Firm: Murray; Thomas H. Poff; Clifford
A.
Claims
We claim as our invention:
1. A system for supporting a bundle of tubes against lateral
movement in a heat exchanger in which a first fluid flows through
the tubes and a second fluid flows past the outer walls of the
tubes so as to transfer heat from one fluid to the other, said
system comprising spring collar devices each surrounding a tube
forming said bundle with adjacent spring collar devices being in
engagement with each other to form a cluster of such spring collar
devices to laterally support the bundle of tubes, each spring
collar device having at least one resilient tube contacting portion
for independent resilient lateral support of a tube, the resilient
tube contacting portion of each spring collar device comprising a
spring tab, tube plate means to support the ends of the tubes for
said first fluid flow through the tubes, and a band encircling the
outer peripheral surface of said cluster to compressively hold said
collar devices together at a spaced relation from said tube plate
means while the spring collar devices permit a limited degree of
lateral movement of each tube against its encircling spring collar
device.
2. The system of claim 1 wherein there are a number of spring
collar devices spaced along the length of each tube, the spring
collar devices being spaced along the respective tubes to provide
groups of spring collar devices whose midpoints lie in successive
planes spaced along the tubes with the collar devices in the
respective planar areas being in abutment with each other.
3. The system of claim 2 wherein said spring collar devices are
shaped to permit a fluid to pass therethrough along the lengths of
the tubes.
4. The system of claim 1 wherein each of said spring collar devices
includes a tube-encircling stamping having portions which abut an
associated tube wall and portions which are spaced from the tube
wall, together with bent portions which interconnect the abutting
and spaced portions to afford radial flexibility to the collar
device.
5. The system of claim 4 wherein each of said spring collar devices
is split about its periphery.
6. The system of claim 4 wherein said bent portions extend
throughout the length of each spring collar device, the bent
portions being interconnected by arcuate lobes.
7. The system of claim 4 wherein said bent portions comprise
radially-inwardly bent tabs having inner ends which abut the wall
of an associated tube.
8. The system of claim 1 wherein said spring collar devices each
includes an inner spring member having portions in abutment with
the wall of a tube defining said resilient tube contacting portion
and a tubular collar surrounding and in abutment with the outer
periphery of said inner spring member.
9. The system of claim 8 wherein said tubular collars of adjacent
tubes in said bundle abut each other.
10. The system of claim 1 wherein each spring collar device is
generally rectangular in cross section and has four essentially
flat walls which abut the walls of spring collar devices of
adjacent tubes in said bundle.
11. The system of claim 10 wherein inwardly-bent portions are
provided at the top and bottom of one wall of the spring collar
device forming steady rests which abut the periphery of an
associated tube wall, the wall of said collar opposite said one
wall having inwardly-bent portions at the top and bottom thereof,
said bent portions forming leaf springs which bear upon the
periphery of said associated tube opposite said one wall.
Description
BACKGROUND OF THE INVENTION
As is known, nuclear power plants of the pressurized-water type
include steam generators in which the steam which drives the
turbine-generator is produced from a secondary coolant by transfer
of heat in a heat exchanger from a closed primary coolant system. A
steam generator of this type consists of a bundle of
vertically-oriented tubes inside a cylindrical shell. In certain
steam generators, the tubes are inverted U-tubes fastened to a
lower tube sheet. In other types of steam generators, the tubes are
straight and are fastened to lower and upper tube sheets. In either
case, the primary coolant from the reactor flows inside the
vertically-oriented tubes and steam is generated in the secondary
coolant outside the tubes and flows upwardly along the tubes within
the cylindrical shell of the steam generator.
The tube bundle is ordinarily many feet high; and, consequently,
the tubes must be supported laterally at several elevations along
their length in order to limit flow-induced vibrations during
normal operation and to limit stresses in the tubes during severe
abnormal conditions such as earthquakes or tube ruptures.
The lateral support means for tubes in existing steam generator
designs consists of a plurality of spaced tube support plates or
egg-crate support grids. In the case of tube support plates,
drilled or broached holes are provided through which the tubes pass
with small diametral clearances. In order to limit fretting at the
supports during operation, the radial clearances between the tubes
and the lateral support device must be small and the tube support
plates or grids are made of a different material than the tubes.
Materials commonly used for lateral supports are carbon steel and
ferritic stainless steel. The tubes are commonly a high nickel
alloy.
Carbon steel has been found to be susceptible to corrosion in the
secondary-side water chemistries of power plants. A result has been
the phenomenon of "tube denting", in which build-up of the
corrosion product (which has larger volume than the base metal)
squeezes the tube at the support location and creates tensile
stresses in the tube support plate or grid. Ferritic stainless
steel is viewed as a means of overcoming tube denting but, based on
limited experimental evidence to date, it is also apparently
susceptible to corrosion in typical secondary-side water
chemistries.
Tube support plates or grids are expensive to construct and, in
part because of the small clearances at the supports, cause
installation of tubes to be somewhat difficult, and consequently
expensive.
SUMMARY OF THE INVENTION
In accordance with the present invention, corrosion of the tube
support plate or grid, the consequent effects of corrosion (e.g.,
denting), and fretting between the tubes of a heat exchanger and
the tube lateral support plates are essentially eliminated by
replacing the support plates or grids with spring collar devices
which surround each tube such that adjacent spring collars are in
abutment with each other to form a cluster of tubes, each
surrounded by a spring collar device. The cluster of tubes is held
together by the compressive effect of an encircling band or some
other device at the periphery of the cluster which abuts against
the spring collars on the outer rows of tubes of the tube
cluster.
Because of the length of the tube bundle, a number of spring collar
devices are spaced along the length of each tube to provide groups
of spring collar devices whose midpoints lie in successive planes
spaced along the tubes with the collar devices in the respective
planar areas being in abutment with each other. With this
arrangement, no relative movement occurs between the spring collar
devices and the tubes themselves; however there can be relative
movement between abutting spring collar devices. Since there is no
movement between the tubes and their supporting spring collar
devices, the potential for fretting is essentially eliminated.
As will be apparent from the subsequent description, the material
of the spring collar can be a high nickel alloy which is less
susceptible to corrosion than the materials used in tube support
plates or grids. Further, the configuration of the spring collar is
such as to reduce the potential for locally high temperatures on
the tube wall and resultant local deposition of corrosion inducing
impurities from the secondary coolant.
The above and other objects and features of the invention will
become apparent from the following detailed description taken in
connection with the accompanying drawings which form a part of this
specification, and in which:
FIG. 1 is a cross-sectional view of a typical steam generator with
which the tube support system of the invention may be used;
FIG. 2 is a partial cross-sectional view taken substantially along
line II--II of FIG. 1;
FIG. 3 is an enlarged cross-sectional view showing on type of
spring collar device which may be utilized in accordance with the
invention;
FIG. 4 is a perspective view of the spring collar device of FIG.
3;
FIG. 5 is a cross-sectional view of another embodiment of the
invention utilizing a modified spring collar device;
FIG. 6A is a perspective view of the spring clip employed in the
embodiment of the invention shown in FIG. 5;
FIG. 6B illustrates the completed spring collar device of FIG. 5
surrounding a tube and utilizing the spring clip of FIG. 6A;
FIG. 7 is a cross-sectional view of still another embodiment of the
invention utilizing a modified spring collar device; and
FIG. 8 is a perspective view of the spring collar device of FIG.
7.
With reference now to the drawings, and particularly to FIG. 1, a
schematic cross section of a typical U-tube steam generator for a
nuclear power plant is shown. It comprises a lower shell 10 and an
upper shell 12. The upper shell 12 normally contains steam
separators and steam driers which form no part of the present
invention and, consequently, are not shown herein. Feed water from
a condenser for a steam turbine, for example, enters the upper
shell 12 through a conduit 14 where it flows downwardly onto a
relatively thick tube sheet 16, typically formed from carbon steel.
Beneath the tube sheet is a semicircular channel head 18 having a
primary coolant inlet 20 and a primary coolant outlet 22 which are
separated by a partition 24 so as to provide two compartments 26
and 28 in the channel head 18.
Contained within the lower shell 10 is a tube bundle comprising a
plurality of U-shaped tubes, only the extreme inner and outer tubes
of which are shown in FIG. 1 and identified by the reference
numerals 30A and 30B. While only a few tubes are shown in FIG. 1,
those skilled in the art will appreciate that the entire volume
within lower shell 10 is filled with U-shaped tubes each having an
end communicating with the chamber 26 and another end communicating
with the chamber 28 so that the entire area of the tube sheet 16
has tubes passing therethrough.
FIG. 2 is a partial cross-sectional view of a completed tube bundle
and includes a plurality of tubes, indicated generally by the
reference numeral 30, which fill the entire cross section of the
lower shell 10. The tubes may be arranged on a square or triangular
pitch. The illustration in FIG. 2 shows a square pitch arrangement.
In FIG. 2, the numeral 36 indicates the outer wall of the lower
shell 10 and the numeral 38 designates a steel or the like baffle
which surrounds the tube bundle to separate the downward flow of
subcooled water from the upward flow of steam and to hold the tubes
30 in assembled relationship.
Reverting again to FIG. 1, primary coolant from the reactor will
enter chamber 26, enter the lower ends 40 of the tubes 30 and then
pass out of the other ends 42 of the same tubes 30 and into chamber
28 which communicates with outlet port 22 leading back to the
nuclear reactor, now shown. Water entering the feed water nozzle 14
will flow downwardly onto the tube sheet 16 and become heated by
the medium flowing through the tubes 30, thereby forming steam
which rises upwardly and into the upper shell 12.
The present invention relates to a system for giving lateral
support to a bundle of tubes, such as tubes 30, which are dispersed
throughout the entire cross section of the lower shell 10. As was
explained above, it has been common in the past to laterally
support the tubes 30 by passing them through openings or holes in
the tube support plates located at the approximate positions of the
lateral supports 32 shown in FIG. 1. This, however, leads to the
problems of fretting at the peripheries of the holes in the support
plates during operation or of galling during tube insertion. It
also results in local high temperatures of the tube wall where the
tube and tube support plate are separated only by a small
clearance, concentration of impurities in the water and consequent
corrosion of the tube support plates which leads to a build-up of
corrosion products and "tube denting" as explained above, or simply
weakening of the tube support plate by loss of metal.
In accordance with the present invention, fretting and corrosion
build-up are prevented by surrounding each of the tubes, at spaced
locations in the lower shell 10, with spring collar devices, one
embodiment of which is illustrated in FIGS. 3 and 4. As shown in
FIG. 3, each of the tubes 30 is surrounded at spaced points along
its length by spring clips or collars 35, the details of which are
shown in FIG. 4. Each spring clip 35 comprises a stamping of
generally square cross section having four side walls 37 which abut
the walls of adjacent spring clips. At the upper and lower ends of
one of the walls 37 are inwardly-bent portions 39 forming steady
rests which abut the periphery of an associated tube wall. At the
top and bottom of the diametrically-opposite wall 37 are
inwardly-bent portions 41 which form leaf springs which bear upon
the opposite side of each tube. As will be understood, any lateral
movement of the tubes 30 will cause a deflection of the leaf
springs 41 which will nevertheless remain in engagement with, and
bear upon, the tube wall. Because of the degree of flexibility
afforded by the springs 41, tube fretting and corrosion build-up
will be eliminated or materially reduced. At the same time, the
spring clips readily permit the upward flow of steam past the tube
walls.
The clips 35 are passed over the tubes 30 and are positioned at
spaced locations with the clips for all tubes at each location or
cluster having their midpoints lying in a common plane. Each
cluster, in turn, is encircled by a band, such as baffle 38 shown
in FIGS. 1 and 2, spaced from the wall 36 of the steam generator by
spacers 61.
Another embodiment of the invention is shown in FIGS. 5, 6A and 6B.
In this case, each spring clip 44 is provided with a split annular
lower portion 46 and upstanding arcuate portions 48 and 50 which
are separated one from the other. The upper parts of the arcuate
portions 48 and 50 are bent inwardly to provide spring tabs 52
which, as shown in FIG. 5, abut the outside wall of each tube 30 on
opposite sides thereof. In this regard, it will be appreciated that
the spring clip 44 comprises a tube-encircling stamping having
portions (i.e., the ends of tabs 52) which abut an associated tube
wall and portions 54 which are spaced from the wall of the tube,
the two portions 52 and 54 being interconnected by bent portions
56. Surrounding the spring clip 44, as best shown in FIGS. 5 and
6B, is a tubular collar 58 which abuts similar collars of adjacent
tubes 30.
In assembling the tube bundle, spring clips 44 and their encircling
collars 58 are passed over the tubes 30 at spaced locations as in
the previous embodiment, with the spaced spring clips along the
length of the tubes all having their midpoints lying in common
planes whereby a band, such band 38 shown in FIGS. 1 and 2, can
encircle all of the tubes and press the tubular collars 58 of
adjacent tubes into engagement with each other. The tubes 30 are
thus permitted a degree of lateral movement against the force of
the spring tabs 52 within each collar 58. At the same time, because
of the flexibility involved, there is little or no relative motion
between each tube 30 and its lateral support. Consequently, no
potential for fretting of the tube exists. That is, all relative
motion is between the spring collar devices. There is also less
restriction to secondary coolant flow parallel to the tubes than is
the case with the prior art tube support plates or grids described
above. As a result, tube wall superheat, which is a cause of
localized deposition of suspended or dissolved solids, and coolant
pressure drop, are both reduced. The preferred material of the
spring clips is INCONEL (Trademark). As was explained above, the
baffle 38 or other compression device encircles the tube bundle as
shown in FIGS. 1 and 2. Several different configurations of
compression devices are possible such as a flexible band, a split
ring, wedges and the like. Lateral forces on the tubes are
transmitted through the baffle 38 or other compression device to
the wall 36 (FIG. 2) of the steam generator through the spacers
61.
With reference now to FIGS. 7 and 8, still another embodiment of
the invention is shown which, like the embodiment of FIGS. 3 and 4,
does not employ a surrounding tubular sleeve, such as collar 58 in
FIG. 6B. In FIGS. 7 and 8, spring collars 60 surround each of the
tubes 30 and comprise radially-outwardly extending arcuate lobes 62
interconnected by radially-inwardly extending portions 64 which
abut the wall of each tube 30. The radii of the lobes 62 have
centers coincident with the central axis of each tube 30 such that
line contact will be made between a lobe 62 and a lobe of an
adjacent tube regardless of the angular relationship of the tubes
30. In FIG. 7, the tubes 30 are shown in exact quadrature; however
it will be appreciated that in an actual installation, and
particularly since the bundle of tubes will have a circular outer
periphery, the tubes may not be in exact quadrature throughout the
cross section of the steam generator. Note also that each of the
spring collars 60 is split as at 66 between adjacent lobes so as to
achieve greater flexibility of each of the spring collars 60. As in
the previous embodiments, the spring collars 60 permit relative
movement between the tubes 30 without fretting and corrosion
build-up. Also, as in the previous embodiments, each spring collar
device includes a tube-encircling stamping having portions 64 which
abut an associated tube wall, portions 62 which are spaced from the
tube wall and bent portions 68 interconnecting the abutting and
spaced portions 64 and 62.
Although the invention has been shown in connection with certain
specific embodiments, it will be readily apparent to those skilled
in the art that various changes in form and arrangement of parts
may be made to suit requirements without departing from the spirit
and scope of the invention.
* * * * *